A lithium mixed metal oxide is used as a positive electrode active material in nonaqueous electrolyte secondary batteries, such as a lithium secondary battery. The lithium secondary battery has already been put into practical use as a power source for portable telephones, notebook-sized personal computers and the like, and also an attempt is made to use the battery in medium and large size applications, such as applications of use for automobiles and electric power storages.
A mixed oxide of lithium and a cobalt-based oxide has hitherto been used as a positive electrode active material of the lithium secondary battery. However, use of oxides of transition metals other than cobalt, such as a nickel-based oxide and a manganese-based oxide has been studied from the viewpoint of reserves and costs of cobalt. For example, JP5-290851A discloses a method for producing a lithium-nickel mixed oxide in which a nickel compound and a lithium compound are mixed, and then heat-treated under an air atmosphere at 600° C. for 20 hours.
Lithium hydroxide is mainly used as a lithium raw material for a lithium mixed metal oxide. However, lithium hydroxide is likely to react with carbon dioxide in the air to form lithium carbonate (Li2CO3). Since lithium carbonate has low reactivity with a transition metal raw material, the formed lithium mixed metal oxide may have a non-uniform composition.
In order to avoid the reaction of lithium hydroxide with carbon dioxide in the atmosphere, in a conventional method for producing a lithium mixed metal oxide, pure oxygen or synthetic air containing no carbon dioxide is used, or calcination is carried out under an air atmosphere where the content of a carbon dioxide gas is about 0.01% by volume or less (see, for example, JP2000-58053A).
As mentioned above, in order to sufficiently react a lithium raw material in a conventional method for producing a lithium mixed metal oxide, it is necessary that an atmosphere controllable calcination furnace is used, or pure oxygen or synthetic air containing no carbon dioxide is used, or air, from which carbon dioxide has been removed by providing a special facility, is used, and thus leading to a factor of high costs.
In the case of producing a large amount of the lithium mixed metal oxide whose demands as a positive electrode active material for a nonaqueous electrolyte secondary battery are rapidly increasing, it is desired to use, as a calcination furnace for obtaining a lithium mixed metal oxide, a versatile gas furnace using flame of a combustion gas such as propane as a heat source from the viewpoint of an improvement in mass productivity of the lithium mixed metal oxide and cost reduction. However, since such a gas furnace undergoes heating by combustion of a hydrocarbon gas such as propane, high concentration of CO2 (usually 10 to 15% by volume, or about 5 to 10% by volume even in the case of providing CO2 removal means) is contained inside the furnace. Therefore, when calcination is performed under a conventional temperature condition using such a gas furnace, as mentioned above, a lithium compound (for example, lithium hydroxide) easily reacts with carbon dioxide to form lithium carbonate having low reactivity. As a result, an insufficiently reacted lithium mixed metal oxide is synthesized, and thus sufficient battery performance can not be obtained even if a nonaqueous electrolyte secondary battery is produced using the lithium mixed metal oxide as a positive electrode active material.
Under these circumstances, an object of the present invention is to provide a method capable of stably producing a lithium mixed metal oxide which can be used as a positive electrode active material of a nonaqueous electrolyte secondary battery even under a calcination atmosphere containing higher concentration of carbon dioxide than that in the air.